Horton

Re: Wrong comment for ReplicationSlotCreate

Chao Li <li.evan.chao@gmail.com>

View thread
From: Chao Li <li.evan.chao@gmail.com>
To: Daniil Davydov <3danissimo@gmail.com>
Cc: Postgres hackers <pgsql-hackers@lists.postgresql.org>
Date: 2025-12-31T02:32:17Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Update comments atop ReplicationSlotCreate.

  2. Fix typo in slot.c.


> On Dec 30, 2025, at 18:07, Daniil Davydov <3danissimo@gmail.com> wrote:
> 
> Hi,
> 
> On Tue, Dec 30, 2025 at 4:18 PM Chao Li <li.evan.chao@gmail.com> wrote:
>> 
>> But I think the updated version is too simple. It loses the information that enabling two_phase later can result in missing PREPARE.
>> 
>> So, I would suggest something like:
>> ```
>> * two_phase: If enabled, allows decoding of prepared transactions.
>> *     Note that enabling this option after decoding has already advanced
>> *     may result in missing PREPARE records for transactions that were
>> *     prepared before the option was enabled.
>> 
>> ```
> 
> Thanks for the review!
> 
> As far as I understand, if the publisher prepares a transaction and
> then subscriber
> tries to create a subscription, walsender will wait until the prepared
> transaction is
> finished (during execution of  CREATE_REPLICATION_SLOT command).
> We can find this logic inside the SnapBuildFindSnapshot function. Thus, we
> cannot miss any PREPARE record for the created slot.
> 
> Am I missing something?
> 
> --
> Best regards,
> Daniil Davydov


You’re right that during CREATE_REPLICATION_SLOT, SnapBuildFindSnapshot() ensures we don’t miss PREPARE records for prepared transactions that exist at creation time.

1462aad2e introduced support for altering logical replication slot options, including two_phase, after the slot has been created. The commit comment says:
```
    Changing the 'two_phase' option for a subscription from 'true' to 'false'
    is permitted only when there are no pending prepared transactions
    corresponding to that subscription. Otherwise, the changes of already
    prepared transactions can be replicated again along with their corresponding
    commit leading to duplicate data or errors.
```

true->false is not allowed, however false->true is permitted. So, I think the old comment is still possible today:
```
*     Note that enabling this option after decoding has already advanced
*     may result in missing PREPARE records for transactions that were
*     prepared before the option was enabled.
```

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/